FR-A- 2 596 197 and corresponding EP-A-0240399 disclose a switch mounted in a casing, in which a driven assembly carrying contacts is abruptly moved in one direction by the expansion of resilient means previously tensioned by the movement of a driving assembly (control push member) which moves parallel to said direction. Transmission means driven by the driving assembly produce a constrained movement of the driven assembly during an additional travel of the driving assembly when the resilient means have not been effective in moving the driven assembly at the end of the initial travel of the driving assembly.
Switches of this kind are usable in installations in which operational reliability or staff safety is dependent upon the breaking of an electric circuit. The resilient means, by their abrupt expansion, break the circuit under optimal conditions (absence of any arcs forming between the contacts). On the other hand, the said resilient means may prove inoperative to break the circuit in the event of a breakage of the resilient means, any jamming, or welding of the contacts if they have been in the closed state for a long time. In that case the transmission means constrain the driven assembly to perform its movement when the driving assembly performs its additional travel.
On installation of such switches it is often necessary to adjust the relative position of the limit switch and the movable member required to act on the driving assembly. The movable member is, for example, a cam fixed to a machine or apparatus component, such cam being required to actuate the limit switch when the component reaches a certain position.
It is fairly complex to position the movable member in such manner that, on the one hand, it can produce normal actuation of the limit switch so that the movable member stops in a reference position and, on the other hand, so that it can give the driving assembly a calibrated additional travel intended for constrained actuation of the driven assembly of the switch, such travel being effected only if the movable member has not stopped in its reference position.
Heretofore, the latter adjustment was carried out by trial and error, for example, by listening to the clicking noise produced on abrupt expansion of the resilient means, and by giving the movable member a slight additional displacement. In other cases, a shim is placed between the movable member and the driving assembly in the maximum driven-in position, the shim thickness being selected on the basis that normal actuation of the switch will take place in the required reference position.
Although a limit switch is already known which has an abrupt operation and positive safety control with display of the position of the driven assembly, it does not allow a check to be made to ensure that the adjustment made enables the driving assembly to perform the additional travel required for constrained actuation in the event of breakdown of the resilient actuating means.